Asphalt is a common material utilized for the preparation of roofing members and coatings which may be applied as mopping grade asphalts, cutbacks in solvents, single ply membranes, shingles, roll roofing membranes, etc. While the material is suitable in many respects, in inherently is deficient in some physical properties which it would be highly desirable to improve. Efforts have been made in this direction by addition of certain conjugated diene rubbers, neoprene, resins, fillers and other materials for the modification of one or more of the physical properties of the asphalt binder. Each of these added materials modifies the asphalt in one respect or another but certain deficiencies can be noted in all compounds proposed. For example, some of them have excellent weather resistance, sealing and bonding properties but are often deficient with respect to warm tack, modulus, hardness and other physical properties.
Since the late 1960s, styrene-butadiene rubber and styrene-rubber block copolymers such as styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers have been used to dramatically improve the thermal and mechanical properties of asphalts. Practical application of the rubber addition approach requires that the blended product retain improved properties and homogenity during transportation, storage and processing. Long term performance of elastomer-modified asphalts also depends on the ability of the blend to maintain thermal and chemical stability.
To be suitable for synthetic roofing materials, the asphalt-block copolymer mixtures should meet the following requirements:
(a) sufficient resistance to flow at high temperatures,
(b) sufficient flexibility at low temperatures,
(c) workability according to the conventional methods used in the roofing technique,
(d) adequate hot storage stability,
(e) adequate hardness to prevent deformation during walking on the roof, and
(f) if it is to be used as an adhesive, sufficient adhesion.
For roll roofing applications, it is preferred that the softening point (the temperature at which the material will tend to flow) be above about 250.degree. F., the cold bend temperature (the temperature at which the material will crack during application and service), which is not as critical a parameter as the others in this application, should be below about -5.degree. C. and that the asphalt and block copolymer components should be able to be mixed and processed at a temperature no higher than about 200.degree. C. to keep the asphalt heating costs down and to prevent softening of the polyester reinforcement commonly used in these membranes.
For roll roofing membranes, the bituminous composition is used to saturate and coat a reinforcing mat. The bitumen is there to make the membrane waterproof. The mat is used to aid in mechanical properties (gives the membrane strength etc.). Polymer is added to the asphalt to improve the weatherability and mechanical properties of the asphalt.
Until recently, only unhydrogenated block copolymers were being used in roll roofing applications. For instance, a linear unhydrogenated styrene-butadiene-styrene block copolymer with a total molecular weight of 110,000 and a polystyrene content of 31% could be used for such applications. When 12% of this block copolymer is used with AC-10 blend asphalt (defined later in the examples), the softening point is about 230.degree. F., the cold bend temperature is about -25.degree. C. and the components can be mixed at a temperature of approximately 160.degree.-180.degree. C. Another unhydrogenated block copolymer, a coupled radial styrene-butadiene block copolymer with a total molecular weight of 264,000 and a polystyrene content of 31%, could also be used in such applications. When blended with the same asphalt at the same concentration, the softening point is approximately 262.degree. F., the cold bend temperature is approximately -25.degree. C. and the components can be mixed at approximately 180.degree.-200.degree. C. Unhydrogenated block copolymers have certain disadvantages which can cause problems when used in applications such as these. Such disadvantages include poor stability of the block copolymer during blending and storage of the bituminous composition and poor long term stability when the bituminous composition is exposed to the elements (by stability we mean resistance to degradation) or heat.
Resistance to degradation under the application of heat is an important consideration in materials for roll roofing membranes. Roll roofing membranes are used, for example, to protect the surface of a roof. The membrane is rolled up and when applied, is merely unrolled in place on the roof. A roll roofing membrane is comprised of a reinforcing mat saturated and coated with asphaltic compositions which may contain a modifying polymer. One application method to secure the membrane to the roof is torching, i.e. heating with a flame at a high temperature, perhaps close to 2000.degree. C. Unhydrogenated block copolymers have a tendency to degrade when exposed to such extreme heat making them less desirable for this application.
High performance roll roofing membranes which comprise a reinforcing mat coated with nonhydrogenated block copolymer modified asphalt can be overtorched. Excessive torching can cause substantial polymer degradation. This can cause a layer of polymer modified asphalt with poor high temperature flow resistance. In other words, it could contribute to roof failure by slippage of the membrane.
It is now known that saturated block copolymers are useful to modify asphalt in roofing applications. For example, copending, commonly assigned U.S. patent application Ser. No. 553,042, filed Jul. 16, 1990, now U.S. Pat. No. 5,051,457, describes a composition for use in roll roofing membrane applications. The composition comprises about 93 to about 87 parts per hundred of a bituminous component having a penetration of less than about 125 (decamillimeters) at 25.degree. C. and from about 7 to about 13 parts per hundred of a hydrogenated block copolymer of a monoalkenyl aromatic hydrocarbon and a conjugated diolefin having a contour arm a molecular weight before hydrogenation of from about 105,000 to about 140,00 and a polystyrene content of from about 25% to about 37%.
The saturated or hydrogenated block copolymers are more expensive than their unsaturated or unhydrogenated counterparts. Therefore, it would be more expensive to utilize hydrogenated block copolymers throughout the asphaltic composition which saturates and coats the reinforcing mat of the roll roofing membrane. Thus, there is a need for a way to protect the roll roofing membrane from polymer degradation without having to utilize a large amount of the higher cost saturated polymer.